How can a Muslim man, married to a Christian woman, with Muslim children, ensure his burial and shrouding according to Islamic Sharia after his death, given the potential opposition from his wife, her family, and the Christian community?

Prayer is a pillar of Islam. You must declare your Islam in official documents to be able to practice your religious rites, such as Hajj, and to be buried with Muslims, unless you fear harm, in which case postponement is permissible with the help of Muslims. If you die certain of your Islam, you will not be held accountable for what the living do after your death. You can also بوصي someone to wash and shroud your body.